Don't process files or folders that begin with an underscore

[mojavelinux]

The user generally doesn't want include files to be rendered separately. A convention often used to hide these files from a document processor is to prefix the file name with an underscore. This plugin should honor that convention.

Consider the following hierarchy:

src/
  asciidoc/
    user-manual.adoc
    _examples/
      example-a.adoc
    faq.adoc
    _license.adoc

Both the faq.adoc and user-manual.adoc files should be processed, but the _examples directory and _license.adoc file should be skipped.